The present invention relates to a display device which utilizes an emission of electrons into a vacuum, and more particularly, to a display device which can enhance the display characteristics by implementing electron emitting sources enabling the stable control of an electron emission quantity and a method of manufacturing the display device.
As a display device which exhibits the high brightness and the high definition, color cathode ray tubes have been widely used conventionally. However, along with the recent request for the higher quality of images of information processing equipment or television broadcasting, the demand for planar displays (panel displays) which are light in weight and require a small space while exhibiting the high brightness and the high definition has been increasing.
As typical examples, liquid crystal display devices, plasma display devices and the like have been put into practice. Further, particularly, as display devices which can realize the higher brightness, it is expected that various kinds of panel-type display devices including a display device which utilizes an emission of electrons from electron emitting sources into a vacuum (hereinafter, referred to as xe2x80x9can electron emission type display devicexe2x80x9d or xe2x80x9ca field emission type display devicexe2x80x9d) and an organic EL display which is characterized by low power consumption will be commercialized.
Among such panel type display devices, as the above-mentioned field emission type display device, a display device having an electron emission structure which was invented by C. A. Spindt et al (for example, see U.S. Pat. No. 3,453,478, Japanese Laid-open Patent Publication 21305/2000), a display device having an electron emission structure of a metal-insulator-metal (MIM) type, a display device having an electron emission structure which utilizes an electron emission phenomenon based on a quantum theory tunneling effect (also referred to as xe2x80x9csurface conduction type electron emitting source, see Japanese Laid-open Patent Publication 21305/2000), and a display device which utilizes an electron emission phenomenon having a diamond film, a graphite film and carbon nanotubes and the like have been known.
FIG. 10 is a schematic view for explaining the fundamental constitution of a field emission type display device. In the drawing, CNT indicates carbon nanotubes mounted on a cathode K and A indicates an anode, wherein a fluorescent material PH is formed on an inner surface of the anode A. By applying a voltage Vs between a control electrode G disposed in the vicinity of the cathode K and the cathode K, electrons e are emitted from the carbon nanotubes CNT, while by applying a high voltage Eb between the cathode K and the anode A, the electrons are accelerated and are made to impinge on the fluorescent material PH so that the fluorescent material PH is excited whereby irradiating colored light L which depends on the composition of the fluorescent material PH.
Then, by controlling an electron emission quantity (including turning on and off of emission) in response to the modulated voltage Vs given to the control electrode G disposed in the vicinity of the cathode K, the magnitude (brightness) of the colored light L can be controlled. Here, by providing a focusing electrode F of a given potential between the control electrode G and the anode A so as to focus the electrons e on the fluorescent material PH, the utilization efficiency of the electrons which excite the fluorescent material PH can be enhanced.
FIG. 11A and FIG. 11B are schematic views showing one constitutional example of a known field emission type display device, wherein FIG. 11A is an exploded perspective view and FIG. 11B is a cross-sectional view of the display device after assembling. In this field emission type display device, a face panel 2 having anodes and fluorescent material layers on an inner surface thereof and a rear panel 1 on which field emission type electron emitting sources and control electrodes are formed are arranged to face each other in an opposed manner, a sealing frame 5 is interposed between inner peripheries of these panels so as to seal a space between them, and the inside which is defined by the face panel 2, the rear panel 1 and the sealing frame 5 is reduced to a low pressure (including a vacuum) lower than an external atmospheric pressure or is evacuated (referred to as xe2x80x9cvacuumxe2x80x9d hereinafter).
As shown in FIG. 11A, in this field emission type display device, the rear panel 1 having a substrate 11 preferably made of glass, alumina or the like and the electron emitting sources and the face panel 2 having a substrate 21 made of a light transmitting material such as glass and fluorescent materials are arranged to face each other in an opposed manner.
The sealing frame 5 made of glass or the like is arranged between the rear panel 1 and the face panel 2. This sealing frame 5 is sealed to the rear panel 1 and the face panel 2 respectively using frit glass or the like.
The electron emitting sources and the control electrodes which are not shown in the drawings are formed on the inner surface of the substrate 11 which constitutes the rear panel 1. Cathode terminals 70 which are pulled out from cathodes constituting the electron emitting sources and control electrode terminals 50 which are pulled out from the control electrodes being disposed by way of an insulation layer 16 with respect to the cathodes are provided to a periphery of the rear panel 1. Further, the anodes and the fluorescent materials not shown in the drawings are formed on an inner surface of the substrate 21 which constitutes the face panel 2.
In the drawing, a dotted line 51 depicted on an upper surface of the substrate 11 of the rear panel 1 indicates positions where the outer periphery of the sealing frame 5 is brought into contact with the upper surface of the substrate 11. An exhaust pipe 6 is provided to the outside of a display region of the rear panel 1 and to the inside of the sealing frame 5, and the inside which is surrounded by respective main surfaces of the rear panel 1, the face panel 2 and the sealing frame 5 is evacuated to a vacuum of 10xe2x88x925 to 10xe2x88x927 Torr, for example, by discharging air from the inside.
The carbon nanotubes (CNT) are extremely fine needle-like carbon compound (in a strict sense, molecules formed by binding carbon atoms in a net form and a columnar form) and are used as electron emitting sources when they are arranged on the cathode wires.
In mounting the carbon nanotubes on the cathode wires, a method which coats and bakes a paste in which carbon nanotubes are mixed, a method which exposes end portions of carbon nanotubes in an electric field space by coating a paste in which nickel is mixed as conductive fillers to carbon nanotubes and baking and polishing, or a method which coats a silver paste in which carbon nanotubes are mixed and bakes such silver paste or the like has been adopted.
However, it has been difficult to firmly fix the carbon nanotubes to the cathode wires and to arrange the carbon nanotubes such that end peripheries of the carbon nanotubes which constitute electron emitting portions are efficiently exposed in the inside of the vacuum. When only the paste formed of the carbon nanotubes is used, the electric resistance between carbon nanotubes which constitute needle-like crystals is large and hence, the carbon nanotubes exhibits the small electron emission ability compared to a case the paste is baked with the conductive fillers made of nickel or the like.
When the paste containing nickel (Ni) or the like is used, it is necessary to remove nickel particles exposed on an uppermost surface by polishing. The polishing of such ultra-fine particles is difficult and hence, the method is not suitable for the mass production. Further, the carbon nanotubes which are exposed to the electric field space are substantially formed of only portions which are protruded from a polished surface so that the electron emitting area cannot be increased as it was expected.
Further, when the silver paste is used, most of carbon nanotubes are embedded in the inside of the silver layer or exposed portions are dissipated in the baking step so that it is difficult to sufficiently make use of the original electron beam emitting characteristics of the carbon nanotubes.
It is necessary for the carbon nanotubes to expose the needle-like end portions thereof in the inside of the vacuum and the carbon nanotubes are required to be firmly held on the cathode electrodes to prevent the removal thereof from the cathode electrodes. However, the conventional technique is not sufficient to commercialize such structures in a display device and this constitutes a problem to be solved.
Accordingly, it is an object of the present invention to provide a display device and a manufacturing method of the same which can solve the above-mentioned problem and can realize the highly efficient electron emission characteristics by ensuring the low resistance which enables the carbon nanotubes to have the enough electron emission ability and by ensuring the exposure of the carbon nanotubes in the inside of a vacuum by fixing the carbon nanotubes on cathode wires such that the carbon nanotubes are not easily removed from the cathode wires.
To achieve the above-mentioned object, a display device and a manufacturing method thereof according to the present invention have following fundamental constitutions.
Typical constitutions of the present invention are described hereinafter. First of all, following constitutions are listed as the constitutions of the display device of the present invention.
(1) In a display device performing a display using electrons emitted from electron emitting sources, each electron emitting source includes a large number of carbon nanotubes and portions of the carbon nanotubes in the vicinity of contact points are bonded to each other by films made of bonding material.
Due to such a constitution, the carbon nanotubes are connected with each other by the films made of bonding material and hence, an assembled body of carbon nanotubes which have distal ends thereof sufficiently exposed and have sufficient strength as the electron emitting sources can be formed.
Further, since the distance between the carbon nanotubes becomes stable, the conductive resistance becomes stable and hence, the stable electron emission ability (emission) can be obtained.
(2) Each film made of bonding material in the constitution (1) includes a film produced by baking bonding material which generates a sol-gel reaction by baking.
Due to such a constitution, a larger number of contact points of the carbon nanotubes can be formed due to the shrinkage at the time of baking. Further, the distance between the carbon nanotubes is narrowed due to the shrinkage and hence, the contact area is increased thus lowering the resistance. Here, silica-based material is preferable as the bonding material which generates the sol-gel reaction.
(3) A film obtained by baking the bonding material which generates the sol-gel reaction in the constitution (2) is a film having conductivity.
Due to such a constitution, the bonding material is formed of material having conductivity and hence, the carbon nanotubes exhibit the low resistance and enhance the electron emission ability thereof. As the conductive material added to the bonding material, particles (ultra-fine particles) made of metal oxide such as ITO or other metal can be used.
(4) The bonding material made of conductive material is provided in the vicinity of the contact points of the carbon nanotubes in the constitution (2).
(5) As the bonding material made of conductive material in the constitution (4), bonding material produced by baking material formed of ultra-fine metal particles coated with resin is used. With respect to this conductive material, resin coated on the ultra-fine metal particles is dissipated and the ultra-fine metal particles remain at the crossing portions of the carbon nanotubes and in the vicinity of the crossing portions so as to connect the carbon nanotubes with each other.
With respect to the bonding material in the constitutions (4) and (5), in addition to the material which forms the films based on the sol-gel reaction, as the conductive material in the constitution (4), the ultra-fine metal particles in the constitution (5) which are coated with resin are mixed into the bonding material and the bonding material is baked so that the ultra-fine metal particles remain in the vicinity of the contact points of the carbon nanotubes. This carbon nanotubes exhibit the stable and low resistance and hence, the electron emission ability can be enhanced.
(6) The bonding material in the constitution (1) is bonding material formed of conductive material. Since the bonding material is conductive, the resistance of the carbon nanotubes is reduced compared to the insulating bonding material so that the resistance becomes stable. The bonding material is not limited to those produced by the sol-gel reaction.
(7) Using the ultra-fine metal particles coated with resin as the bonding material made of conductive material in (6), the wettability of the bonding material with the carbon nanotubes is improved. Further, the resin coating is decomposed by baking so that the ultra-fine particles made of metal function as the bonding material and hence, it is possible to produce the bonding material which exhibits the more stability and the less resistance compared with the insulating bonding material.
(8) The electron emitting sources in the constitutions (1) to (7) are formed on conductive cathode wires and portions of the carbon nanotubes are embedded into the cathode wires.
(9) The cathode wires in the constitution (8) include conductive material which is melted or sintered in a temperature range of 150 degree centigrade to 600 degree centigrade.
Since at least the surfaces of the cathode wires are melted or sintered and portions of the carbon nanotubes are embedded into the cathode wires in the baking process, the removal of the carbon nanotubes can be suppressed. To be more specific, the cathode wires contain the conductive material which is melted or sintered in a temperature range of 150 degree centigrade to 600 degree centigrade as constituted in the constitution (9).
Due to the above-mentioned respective constitutions, it is possible to obtain the display device having the highly efficient electron emission ability by fixing the carbon nanotubes to the cathode wires with low resistance and in a state that the carbon nanotubes are not easily removed from the cathode wire and by surely exposing the distal ends of carbon nanotubes in the inside of the vacuum.
Further, as the manufacturing method of a display device, following constitutions can be listed.
(10) In one method, at the time of forming electron emitting sources of the display device which performs a display by utilizing electrons emitted from the electron emitting sources having carbon nanotubes formed on cathode wires, a paste containing the carbon nanotubes is applied to the cathode wires and, thereafter, bonding material which generates a sol-gel reaction by baking is applied to the paste, and the paste and the bonding material are baked.
In this method, the bonding material for the carbon nanotubes is produced by using the so-called sol-gel reaction and hence, contact portions between the carbon nanotubes are increased in number due to the shrinkage at the time of baking whereby the assembly of the carbon nanotubes having the strength can be formed as the electron emitting sources by bonding the contact portions. Further, since the distance between the carbon nanotubes becomes stable, the conductive resistance becomes stable and the stable electron emission ability is obtained.
(11) Conductive material is added to a solution made of the bonding material which generates the sol-gel reaction by baking in the constitution (10). Since the films formed of the bonding material have the conductivity, the carbon nanotubes exhibit the small resistance. Accordingly, the carbon nanotubes can enhance the electron emission ability.
(12) In manufacturing a display device which performs a display using electrons emitted from electron emitting sources having carbon nanotubes formed on cathode wires, a paste which contains the carbon nanotubes and ultra-fine metal particles coated with resin is applied to cathode wires and, thereafter, the paste is baked.
With the use of the paste containing ultra-fine metal particles coated with resin, the wettability of the paste with the carbon nanotubes can be improved. Further, the resin coating is decomposed by baking and the ultra-fine metal particles function as the bonding material and hence, it is possible to produce the more stable and less resistant bonding material compared with the insulating bonding material.
(13) A solution of bonding material which generates a sol-gel reaction by baking is applied between the applying of the paste and the baking of the paste in the constitution (12).
In addition to the bonding of the carbon nanotubes due to the ultra-fine metal particles coated with resin, the contact between the carbon nanotubes is increased due to the shrinkage of the films formed by the sol-gel reaction whereby it is possible to constitute the low-resistant electron emitting sources.
(14) Conductive material is added into the solution of bonding material which generates a sol-gel reaction by baking in the constitution (13).
By adding the conductive material into the solution of bonding material which generates the sol-gel reaction, it is possible to constitute the electron emitting sources which exhibit less resistance compared to the electron emitting sources in the constitution (13).
(15) A quantity of the bonding material in the constitutions (10) to (14) is a quantity which is sufficient for forming films in the vicinity of contact points of the carbon nanotubes after baking.
When a quantity of the bonding material is excessively large, the bonding material wraps the whole carbon nanotubes and considerably reduces the formation of the distal ends of the carbon nanotubes which constitute electron emitting sources. On the other hand, when a quantity of the bonding material is excessively small, it is difficult to bond the contact points of the carbon nanotubes sufficiently. Accordingly, a quantity of the bonding material is set to a quantity which allows the bonding material to form films in the vicinity of the contact points of the carbon nanotubes.
(16) The cathode wires in the constitutions (10) to (14) contain material which is melted or sintered at a temperature for baking the paste containing the carbon nanotubes and the paste is applied to the material which is melted or sintered.
Due to such a method, the carbon nanotubes can make portions thereof embedded into the cathode wires in the baking step and hence, the removal of the carbon nanotubes can be suppressed.
Due to the above-mentioned respective manufacturing methods, it is possible to obtain the display device having the highly efficient electron emission ability by fixing the carbon nanotubes to the cathode wires with low resistance and in a state that the carbon nanotubes are not easily removed from the cathode wire and by surely exposing the distal ends of the carbon nanotubes in the inside of the vacuum.
Here, the present invention is not limited to the above-mentioned constitutions and the constitutions of embodiments which will be explained hereinafter and various modifications are conceivable without departing from the technical concept of the present invention.